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Engine keeps firing with the ignition switch off

Aviation Mechanic Powerplant (AMP) practice question 1 of 11 on Ignition and Starting Systems. ACS element AM.III.H.R2 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONDuring the ignition switch check, the engine keeps firing when the switch is turned to the off position. What does this indicate?

  1. AThe breaker points are stuck closed
  2. BA magneto ground lead is shorted
  3. CA magneto ground lead is open
Show the answer and explanation

Correct answer: C. A magneto ground lead is open

  • A — incorrect The switch check is made to verify that the magneto ground leads are electrically grounded; a failure there points to the ground lead, not the points.
  • B — incorrect A grounded, continuous ground lead is exactly what shuts the magneto off; the fault indicated is an open lead, not a short.
  • C — correct An open magneto ground lead, commonly called the P lead, leaves the magneto operating even with the switch off, and the condition must be corrected.

BACKGROUND

The ignition switch differs from other switches in that the off position completes a circuit to ground rather than opening it, short-circuiting the breaker points so no high voltage can be induced. The ignition switch check is performed to confirm that all magneto ground leads are electrically grounded, usually at about 700 rpm, by momentarily turning the switch off and watching for the engine to quit firing. If the engine does not cease firing in the off position, the magneto ground lead, commonly called the P lead, is open and one or more magnetos are not being shut off. Turning the propeller of an engine in this condition can result in injury or death, because the engine can start with personnel in the propeller arc.

STUDY TIP

Treat any propeller as live until the P leads have been proven grounded, and never move a propeller on an engine that failed the switch check.

KEY POINTS

  • Continued firing with the switch off means an open magneto ground lead.
  • The ignition switch grounds the primary circuit in the off position.
  • A magneto that will not shut off can start the engine when the propeller is turned.

SOURCES

  • sources/amp-powerplant/ch04.txt "Ignition Switch Check"
  • sources/amp-powerplant/ch04.txt "Ignition Switches"
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MORE ON IGNITION AND STARTING SYSTEMS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.